Mirroring multiple writeable storage arrays

ABSTRACT

Systems, methods, and computer program products for mirroring dual writeable storage arrays are provided. Various embodiments provide configurations including two or more mirrored storage arrays that are each capable of being written to by different hosts. When commands to write data to corresponding mirrored data blocks within the respective storage arrays are received from different hosts at substantially the same time, write priority for writing data to the mirrored data blocks is given to one of the storage arrays based on a predetermined criterion or multiple predetermined criteria.

FIELD OF THE INVENTION

The present invention generally relates to computing systems, and moreparticularly relates to systems, methods, and computer program productsfor mirroring dual writeable storage arrays.

BACKGROUND OF THE INVENTION

Mirroring data in two or more data storage systems is used in manyindustries to protect the data in the event that one of the data storagesystems is destroyed or becomes temporarily or permanently inoperable.In many systems, several hosts are capable of writing data to an“active” or read/write data storage system. As data is written to theactive storage array, the data is also substantially simultaneouslywritten to one or more “passive” or read only data storage systems incommunication with the active data storage system. While thisconfiguration is an effective way of mirroring data, the fact that thesesystems only include one active data storage system prevents thesesystems from operating more efficiently.

SUMMARY OF THE DESCRIBED EMBODIMENTS

A fundamental tenet of mirrored technology is to ensure that the datablock in a mirrored pair will substantially always include identicaldata as its peer block in the paired storage system at the end of eachwrite operation. As such, it is therefore desirable to provide methods,systems, and computer program products for mirroring multiple writeablestorage arrays. Furthermore, other desirable features andcharacteristics of the present invention will become apparent from thesubsequent detailed description of the invention and the appendedclaims, taken in conjunction with the accompanying drawings and thisbackground of the invention.

Various embodiments provide a system for mirroring dual writeablestorage arrays. One system comprises a first host computing device, asecond host computing device coupled to the first host computing device,the first host computing device and the second host computing devicearranged in a clustered file system configuration, a first storage arraycoupled to the first host computing device, and a second storage arraycoupled to the first storage array and the second host computing device.In this embodiment, the first storage array and the second storage arrayinclude corresponding blocks for storing data and mirror the data in theother respective corresponding block. Furthermore, the first storagearray and the second storage array are configured to be simultaneouslywritten to by the first host computing device and the second hostcomputing device, respectively, and priority is given to the firststorage array or the second storage array based on a predeterminedcriteria when the first storage array and the second storage arraydesire to simultaneously write data to a same respective correspondingblock.

Other embodiments provide a method for mirroring data in dual writeablestorage arrays. One method comprises receiving a first command, at afirst storage array including a first data storage block, to write firstdata to the first data storage block and receiving a second command, ata second storage array including a second data storage block, to writesecond data to the second data storage block, the first data storageblock and the second data storage block configured to mirror oneanother. The method further comprises writing one of the first data andthe second data to both the first data storage block and the second datastorage block based on a predetermined priority when the first commandand the second command are received substantially simultaneously.

In further embodiments, computer program products for facilitating datacompression are provided. One computer program product comprises acomputer-readable storage medium having computer-readable program codeportions stored therein. In one embodiment, the computer-readableprogram code portions comprise a first executable portion for writingfirst data to a first data storage block within a first storage array inresponse to receiving a first write command and a second executableportion for writing second data to a second data storage block within asecond storage array in response to receiving a second write command,the first data storage block and the second data storage blockconfigured to mirror one another. The computer-readable program codeportions further comprise a third executable portion for writing one ofthe first data and the second data to both the first data storage blockand the second data storage block based on a predetermined priority whenthe first write command and the second write command a receivedsubstantially simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a block diagram of one embodiment of a system for mirroringdata in a dual writeable storage array configuration; and

FIG. 2 is a flow diagram of one embodiment of a method for mirroringdata in a dual writeable storage array configuration.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

Turning now to the figures, FIG. 1 is a block diagram of one embodimentof a system 100 for mirroring data in a dual writeable storage arrayconfiguration. At least in the illustrated embodiment, system 100comprises a local area network (LAN) 105 coupled to at least two hostcomputing devices (e.g., host 110 and 115) arranged as a clustered filedsystem. System 100 further comprises a data storage system 120 and adata storage system 130 coupled to host 110 and host 115, respectively,and are in communication with one another.

LAN 105 may be any type of local area network known in the art ordeveloped in the future. Likewise, hosts 110 and 115 may be any type ofcomputing device and/or system capable of functioning as a hostcomputing device and/or system in a clustered file configuration.

Data storage system 120 comprises a cache memory 1210 and a plurality ofblocks of storage or memory 1215 arranged in an array 1220 having one ormore tracks 1222 (or rows) and one or more sectors 1224 (or columns)within one or more dimensions (e.g., pages, chapters, sections, folders,books, etc.). Data storage system 120 may include any amount of memoryknown in the art or developed in the future. That is, array 1220 mayinclude any number of blocks of memory 1215, each of which may includeany size that is known in the art or developed in the future. Array 1220may refer to one or more physical, logic, or virtualized storageelements coupled together to present a plurality of memory addressableusing terminology such as logical unit numbers (LUNs), cylinders,tracks, and sectors. Array 1220 may be formed of the same number oftracks and sectors of blocks of memory 1215 or may have a differentnumber of tracks and sectors of blocks of memory 1215 as illustrated inthe embodiment depicted in FIG. 1. As discussed above, data storagesystem 120 is configured to be in communication with data storage system130. Data storage system 120 may include additional storage or memorynot involved in the mirroring relationship between array 1220 and array1320.

Data storage system 130 comprises a cache memory 1310 and a plurality ofblocks of storage or memory 1315 arranged in an array 1320 having one ormore tracks 1322 (or rows) and one or more sectors 1324 (or columns)within one or more dimensions (e.g., pages, chapters, sections, folders,books, etc.). Specifically, data storage system 130 includes the samenumber of blocks of memory 1315 as data storage system 120 and eachblock of memory 1315 in array 1320 includes a corresponding block ofmemory 1215 in array 1220 such that the data in the corresponding blocksof memory are mirrored in arrays 1220 and 1320. As such, array 1320includes the same number of tracks 1322 and sectors 1324 as tracks 1222and sectors 1224, respectively, in array 1220. Data storage array mayalso include additional storage or memory not involved in the mirroringrelationship between array 1220 and array 1320.

With the arrangement discussed above, system 100 is able tosubstantially simultaneously write data to storage systems 120 and 130while mirroring the data in the other respective data storage system.Specifically, as data is being written to a particular block of memory1215 in array 1220 by host 110, the data is also being written (i.e.,mirrored by data storage system 120) to data storage system 130 withinstructions for the corresponding block of memory 1315 in array 1320 tobe updated with the new data. Likewise, as data is being written to aparticular block of memory 1315 in array 1320 by host 115, the data isalso being written (i.e., mirrored by data storage system 130) to datastorage system 120 with instructions for the corresponding block ofmemory 1215 in array 1220 to be updated with the new data. Tosubstantially simultaneously write data to storage systems 120 and 130,caches 1210 and 1310 are configured to cooperate with one another inwriting and mirroring data to their respective arrays 1220 and 1320.

Caches 1210 and 1310 are the components of the respective data storagesystems 120 and 130 providing the function of caching data for rapidaccess. This function includes the physical memory, process code, andcentral processing unit tasked with maintaining this function. Coupledwith this function, is peer to peer mirroring code to ensure that alldata writes to locally paired data are also successfully written to theconfigured peer storage system. It is this function that implements thedesign of the present invention to enable multiple writable mirroredvolumes. Subsequent references to caches 1210 and 1310 imply thisdefinition.

Cache 1210 is configured to receive commands to write data variousblocks of memory 1215 in array 1220. Cache 1210 is further configured toinstruct cache 1310 to write (i.e., mirror) the data in the variouscorresponding blocks of memory 1315 in array 1320 upon receipt of thecommands from host 110. Cache 1310 is configured to transmitacknowledgements of the instructions to cache 1210 and write the data tothe various corresponding blocks of memory 1315 in array 1320 uponreceipt of the instructions from cache 1210. Cache 1210 is configured towrite the data to the various blocks of memory 1215 in array 1220 uponreceipt of the acknowledgements of the instructions from cache 1310.Cache 1210 is then configured to transmit acknowledgements to host 110after the data has been written to array 1220 and acknowledgements thatthe data has been written (i.e., mirrored) in array 1320 is receivedfrom cache 1310. Caches 1310 and 1210 operate similarly when cache 1310receives a command to write data to array 1320 from host 115.

Similar to cache 1210, cache 1310 is configured to receive commands towrite data various blocks of memory 1315 in array 1320. Cache 1310 isfurther configured to instruct cache 1210 to write (i.e., mirror) thedata in the various corresponding blocks of memory 1215 in array 1220upon receipt of the commands from host 115. Cache 1210 is configured totransmit acknowledgements of the instructions to cache 1310 and writethe data to the various corresponding blocks of memory 1215 in array1220 upon receipt of the instructions from cache 1310. Cache 1310 isconfigured to write the data to the various blocks of memory 1315 inarray 1320 upon receipt of the acknowledgements of the instructions fromcache 1210. Cache 1310 is then configured to transmit acknowledgementsto host 115 after the data has been written to array 1320 andacknowledgements that the data has been written (i.e., mirrored) inarray 1220 is received from cache 1210. In the event that data from bothhost 110 and 115 are simultaneously destined to be written to the samerespective corresponding block of memory in arrays 1220 and 1320, caches1210 and 1310 implement a predetermined priority to determine which ofthe data from host 110 or host 115 is to be successfully written to thecorresponding blocks of memory. This is done to ensure that the tenet ofensuring the corresponding data blocks 1215 and 1315 in arrays 1210 and1310, respectively, remain mirrored is observed, thus making the twoblocks of memory behave essentially as one block, although they arephysically separate from one another.

In one embodiment, data storage system 120 or data storage system 130will always have priority when commands to write data to the samecorresponding block of memory in arrays 1220 and 1320 are simultaneouslyreceived. That is, data from host 110 will always be written to theblock of memory 1215 in array 1220 and mirrored in the correspondingblock of memory 1315 in array 1320 if data storage system 120 always haspriority. Likewise, data from host 115 will always be written to theblock of memory 1315 in array 1320 and mirrored in the correspondingblock of memory 1215 in array 1220 if data storage system 130 always haspriority.

In another embodiment, the last data storage system to successfullywrite data to any block or dimension of memory, which is tracked (e.g.,via a flag), has priority when commands to write data to the samecorresponding block of memory in arrays 1220 and 1320 are simultaneouslyreceived. In this embodiment, data from host 110 will always be writtento the block of memory 1215 in array 1220 and mirrored in thecorresponding block of memory 1315 in array 1320 if data storage system120 was the last data storage system to successfully write data to anyof blocks of memory 1215. Likewise, data from host 115 will always bewritten to the block of memory 1315 in array 1320 and mirrored in thecorresponding block of memory 1215 in array 1220 if data storage system130 was the last data storage system to successfully write data to anyof blocks of memory 1315.

In yet another embodiment, the last data storage system to successfullywrite data to its corresponding block of memory, which is tracked (e.g.,via a flag), has priority when commands to write data to the samecorresponding block of memory in arrays 1220 and 1320 are simultaneouslyreceived. In this embodiment, data from host 110 will always be writtento the block of memory 1215 in array 1220 and mirrored in thecorresponding block of memory 1315 in array 1320 if data storage system120 was the last data storage system to successfully write data to itscorresponding block of memory 1215. Likewise, data from host 115 willalways be written to the block of memory 1315 in array 1320 and mirroredin the corresponding block of memory 1215 in array 1220 if data storagesystem 130 was the last data storage system to successfully write datato its corresponding block of memory 1315.

In still another embodiment, the last data storage system tosuccessfully write data to the track or dimension that includes itscorresponding block of memory, which is tracked (e.g., via a flag), haspriority when commands to write data to the same corresponding block ofmemory in arrays 1220 and 1320 are simultaneously received. Here, datafrom host 110 will always be written to the block of memory 1215 inarray 1220 and mirrored in the corresponding block of memory 1315 inarray 1320 if data storage system 120 was the last data storage systemto have previously successfully written to any data in the trackincluding its corresponding block of memory 1215. Likewise, data fromhost 115 will always be written to the block of memory 1315 in array1320 and mirrored in the corresponding block of memory 1215 in array1220 if data storage system 130 was the last data storage system to havepreviously successfully written to any data in the track including itscorresponding block of memory 1315.

In still yet another embodiment, the last data storage system tosuccessfully write data to the sector or dimension that includes itscorresponding block of memory, which is tracked (e.g., via a flag), haspriority when commands to write data to the same corresponding block ofmemory in arrays 1220 and 1320 are simultaneously received. In thisembodiment, data from host 110 will always be written to the block ofmemory 1215 in array 1220 and mirrored in the corresponding block ofmemory 1315 in array 1320 if data storage system 120 was the last datastorage system to successfully write data to the sector or dimensionincluding its corresponding block of memory 1215. Likewise, data fromhost 115 will always be written to the block of memory 1315 in array1320 and mirrored in the corresponding block of memory 1215 in array1220 if data storage system 120 was the last data storage system tosuccessfully write data to the sector or dimension including itscorresponding block of memory 1315.

Priority, in another embodiment, is based on a permission scheme. Inthis embodiment, data storage system 120 and data storage system 130 arearranged in a master/slave configuration. Here, the slave data storagesystem is required to ask for and receive permission from the masterstorage system prior to writing any data. In this manner, when commandsto write data to the same corresponding block of memory in arrays 1220and 1320 are simultaneously received, the master storage system is ableto grant or deny permission to write data based on any predeterminedfactor as long as the tenet of ensuring the corresponding blocks ofmemory are identical is always adhered to.

Priority, in yet another embodiment, is based on a de facto priorityscheme. That is, one of data storage systems 120 and 130 functions as adeferring data storage system and will defer initial processing ofreceived write commands to the other non-deferring data storage system,which then processes the write command as if the write command werereceived from a host computer. Specifically, if data storage system 130is the deferring data storage system, data storage system 130 isconfigured to transmit commands to write any data to various blocks ofmemory 1315 in array 1320 from host 115 to data storage system 120 forinitial processing. Here, cache 1210 receives data from data storagesystem 130 as if storage data system 130 is a host. Cache 1210, thenon-deferring cache, is configured to then instruct cache 1310 to writethe data in the various blocks of memory 1315 in array 1320 upon receiptof the commands from data storage system 130. Cache 1310 is configuredto transmit acknowledgements of the instructions to cache 1210 and writethe data to the various blocks of memory 1315 in array 1320 upon receiptof the instructions from cache 1210. Cache 1210 is configured to write(i.e., mirror) the data to the various corresponding blocks of memory1215 in array 1220 upon receipt of the acknowledgements of theinstructions from cache 1310. Cache 1310 is further configured totransmit acknowledgements to cache 1210 once the data has been writtenlocally. Cache 1210 is then configured to transmit acknowledgements todata storage system 130 indicating that the data has been written toarray 1220 and that the data has been written (i.e., mirrored) to array1320. Data storage system 130 is then configured to transmitacknowledgements to host 115 indicating that the data has been writtenand mirrored. Data storage system 120 is also capable of functioning asa deferring data storage system.

When data storage system 120 is the deferring data storage system, datastorage system 120 is configured to transmit commands to write data tovarious blocks of memory 1215 in array 1220 from host 110 to datastorage system 130 for initial processing. Here, cache 1310 receivesdata from data storage system 120 as if data storage system 120 is ahost. Cache 1310, the non-deferring cache, is configured to theninstruct cache 1210 to write the data in the various blocks of memory1215 in array 1220 upon receipt of the commands from data storage system120. Cache 1210 is configured to transmit acknowledgements of theinstructions to cache 1310 and write the data to the various blocks ofmemory 1215 in array 1220 upon receipt of the instructions from cache1310. Cache 1310 is configured to write the data to the variouscorresponding blocks of memory 1315 in array 1320 upon receipt of theacknowledgements of the instructions from cache 1210. Cache 1210 isfurther configured to transmit acknowledgements to cache 1310 once thedata has been written locally. Cache 1310 is then configured to transmitacknowledgements to data storage system 120 indicating that the data hasbeen written to array 1320 and that the data has been written (i.e.,mirrored) to array 1220. Data storage system 120 is then configured totransmit acknowledgements to host 110 indicating that the data has beenwritten to array 1220 and mirrored.

After priority policy has been established, system 100, in oneembodiment, is configured to delete/ignore the write commands to thedata storage system (or host) that does not have priority whenprocessing write commands to the same blocks at substantially the sametime. In this embodiment, the last write activity (and therefore themost recent data) in the corresponding memory blocks will belong to thedata storage system that did have priority as only the priority writeswere processed.

In still another embodiment, the write commands from the data storagesystem (or host) that does not have priority may be delayed processinguntil after the write commands from the data storage system (or host)that has priority have been processed (i.e., written to and mirrored inarrays 1220 and 1320). In this embodiment, the last write activity (andtherefore the most recent data) in the corresponding memory blocks willbelong to the data storage system that did not have priority as thepriority writes were processed first. Either way, the fundamental tenantto ensure that both mirrored blocks have identical data after everywrite operation is being adhered to.

Turning now to FIG. 2, is a flow diagram of one embodiment of a method200 for mirroring data in a dual writeable storage array system (e.g.,system 100). At least in the illustrated embodiment, method 200 beginsby receiving a first command, at a first cache (e.g., cache 1210), froma first host (e.g., host 110) to write data to a first block of memory(e.g., a block of memory 1215) in a first array (e.g., array 120) andmirror the data in a corresponding second block of memory (e.g., a blockof memory 1315) in a second array (e.g., array 130) (block 210), andreceiving a second command, at a second cache (e.g., cache 1310), from asecond host (e.g., host 115) to write different data to the second blockof memory 1315 and mirror the data in the corresponding first block ofmemory 1215 at substantially the same time (block 220).

When the first command and second command are received at substantiallythe same time, method 200 includes determining which of the firstcommand and the second command has priority based on predeterminedfactors (block 230). In one embodiment, the first command or the secondcommand will always have priority because it is being written to firstarray or second array, respectively. In another embodiment, the firstcommand or the second command will always have priority because thecommands are from host 110 or host 115, respectively.

Priority, in other embodiments, is given to the first command or thesecond command based on which of array 120 or 130 was last written to,which of block 1215 or 1315 was last written to, which track includingblock 1215 or 1315 was last written to, which sector including block1215 or 1315 was last written to, and/or the like factors discussedabove. In yet another embodiment, priority is based on a permissionscheme between arrays 120 and 130 similar to the discussion above. Instill another embodiment, priority is based on a de facto scheme betweenarrays 120 and 130 similar to the discussion above.

After priority is determined, method 200 includes writing the data inthe first command to block of memory 1215 and mirroring the data in thefirst command in block of memory 1315, or writing the data in the secondcommand to block of memory 1315 and mirroring the data in the secondcommand in block of memory 1215 (block 240). Method 200 furthercomprises either discarding whichever of the first command and thesecond command that was determined to not have priority or writing thenon-priority data and mirroring it after the priority data has beenwritten and mirrored (block 250).

An optimization of the method in block 240 may include either writingthe priority data or the non-priority data based on the selection policydetermined in block 230. Subsequently, in block 250, the data block thatwas not selected for writing and mirroring will be discarded. Method 200concludes when the writing and mirroring activities have concluded andhave been acknowledged to the host computer(s) (block 260).

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

As will be appreciated by one of ordinary skill in the art, aspects ofthe present invention may be embodied as a system, method, or computerprogram product. Accordingly, aspects of the present invention may takethe form of an entirely hardware embodiment, an entirely softwareembodiment (including firmware, resident software, micro-code, etc.) oran embodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module,” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, crystal, polymer, electromagnetic, infrared, orsemiconductor system, apparatus, or device, or any suitable combinationof the foregoing. More specific examples (a non-exhaustive list) of thecomputer readable storage medium would include the following: anelectrical connection having one or more wires, a portable computerdiskette, a hard disk, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), an optical fiber, a portable compact disc read-only memory(CD-ROM), an optical storage device, a magnetic storage device, or anysuitable combination of the foregoing. In the context of this document,a computer readable storage medium may be any tangible medium that cancontain, or store a program or data for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wired, optical fiber cable, radio frequency (RF), etc., or any suitablecombination of the foregoing. Computer program code for carrying outoperations for aspects of the present invention may be written in anycombination of one or more programming languages, including an objectoriented programming language such as Java, Smalltalk, C++ or the likeand conventional procedural programming languages, such as the “C”programming language or similar programming languages. The program codemay execute entirely on the user's computer, partly on the user'scomputer, as a stand-alone software package, partly on the user'scomputer and partly on a remote computer or entirely on the remotecomputer or server. In the latter scenario, the remote computer may beconnected to the user's computer through any type of network, orcommunication system, including, but not limited to, a local areanetwork (LAN) or a wide area network (WAN), Converged Network, or theconnection may be made to an external computer (for example, through theInternet using an Internet Service Provider).

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks. The computer program instructions may also beloaded onto a computer, other programmable data processing apparatus, orother devices to cause a series of operational steps to be performed onthe computer, other programmable apparatus or other devices to produce acomputer implemented process such that the instructions which execute onthe computer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the above figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

While one or more embodiments of the present invention have beenillustrated in detail, one of ordinary skill in the art will appreciatethat modifications and adaptations to those embodiments may be madewithout departing from the scope of the present invention as set forthin the following claims.

1. A system for mirroring dual writeable storage arrays, comprising: afirst host computing device; a second host computing device coupled tothe first host computing device, the first host computing device and thesecond host computing device arranged in a clustered file systemconfiguration; a first storage array coupled to the first host computingdevice; and a second storage array coupled to the first storage arrayand the second host computing device, wherein: the first storage arrayand the second storage array include corresponding blocks for storingdata and mirroring the data in the other respective corresponding block,the first storage array and the second storage array are configured tobe written to by the first host computing device and the second hostcomputing device, respectively, and priority is given to the firststorage array or the second storage array based on a predeterminedcriterion when the first storage array and the second storage arraydesire to simultaneously write the data to a same respectivecorresponding block.
 2. The system of claim 1, wherein the predeterminedcriterion is whichever of the first storage array and the second storagearray that was last written to.
 3. The system of claim 1, wherein thepredetermined criterion is whichever of the first storage array and thesecond storage array that last wrote to its respective correspondingblock.
 4. The system of claim 1, wherein: the first storage array andthe second storage array comprise respective corresponding tracksincluding its respective corresponding block, and the predeterminedcriterion is whichever of the first storage array and the second storagearray includes the respective corresponding track that was last writtento.
 5. The system of claim 1, wherein: the first storage array and thesecond storage array comprise respective corresponding sectors includingits respective corresponding block, and the predetermined criterion iswhichever of the first storage array and the second storage arrayincludes the respective corresponding sector that was last written to.6. The system of claim 1, wherein the predetermined criterion is thefirst storage array always has priority or the second storage arrayalways has priority.
 7. A method for mirroring data in dual writeablestorage arrays, comprising: receiving a first command, at a firststorage array including a first data storage block, to write first datato the first data storage block; receiving a second command, at a secondstorage array including a second data storage block, to write seconddata to the second data storage block, the first data storage block andthe second data storage block configured to mirror one another; andwriting one of the first data and the second data to both the first datastorage block and the second data storage block based on a predeterminedpriority or a predetermined policy when the first command and the secondcommand are received substantially simultaneously.
 8. The method ofclaim 7, further comprising discarding the one of the first data and thesecond data that was not written to both the first data storage blockand the second data storage block based on the predetermined priority orthe predetermined policy.
 9. The method of claim 8, wherein writingcomprises always writing the first data from the first data storageblock and discarding comprises always discarding the second data fromthe second data storage block.
 10. The method of claim 7, whereinwriting comprises always discarding the first data from the first datastorage block or always discarding the second data from the second datastorage block.
 11. The method of claim 7, wherein: the first datastorage block is included within a first track or first dimension ofdata blocks within the first storage array and the second data storageblock is included within a second track or second dimension of datablocks within the second storage array; and writing comprises writingthe first data if the first track or first dimension was last written toor writing the second data if the second track or second dimension waslast written to.
 12. The method of claim 7, wherein: the first datastorage block is included within a first sector or first dimension ofdata blocks within the first storage array and the second data storageblock is included within a second sector or second dimension of datablocks within the second storage array; and writing comprises writingthe first data if the first sector or first dimension was last writtento or writing the second data if the second sector or second dimensionwas last written to.
 13. The method of claim 7, wherein writingcomprises writing and mirroring the first data if the first storagearray was last written to or writing and mirroring the second data ifthe second storage array was last written to.
 14. The method of claim 7,wherein writing comprises discarding the first data and mirroring thesecond data if the first storage array was last written to or discardingthe second data and mirroring the first data if the second storage arraywas last written to.
 15. The method of claim 7, wherein writingcomprises writing and mirroring the first data if the first data storageblock was last written to or writing and mirroring the second data ifthe second data storage block was last written to.
 16. The method ofclaim 7, wherein writing comprises discarding the first data andmirroring the second data if the first data storage block was lastwritten to or discarding the second data and mirroring the first data ifthe second data storage block was last written to.
 17. The method ofclaim 7, further comprising: after writing the one of the first data andthe second data, writing the other respective one of the first data andthe second data to both the first data storage block and the second datastorage block when the first command and the second command are receivedsubstantially simultaneously.
 18. A computer program product forfacilitating data compression, the computer program product comprising acomputer-readable storage medium having computer-readable program codeportions stored therein, the computer-readable program code portionscomprising: a first executable portion for writing first data to a firstdata storage block within a first storage array in response to receivinga first write command; a second executable portion for writing seconddata to a second data storage block within a second storage array inresponse to receiving a second write command, the first data storageblock and the second data storage block configured to mirror oneanother; a third executable portion for writing one of the first dataand the second data to both the first data storage block and the seconddata storage block based on a predetermined priority when the firstwrite command and the second write command are received substantiallysimultaneously.
 19. The computer program product of claim 18, whereinthe predetermined priority comprises always writing the first data oralways writing the second data.
 20. The computer program product ofclaim 18, wherein: the first data storage block is included within afirst track and a first sector of data blocks within the first storagearray; the second data storage block is included within a second trackand a second sector of data blocks within the second storage array; andthe predetermined priority comprises one of: writing the first data ifthe first track was last written to or writing the second data if thesecond track was last written to, and writing the first data if thefirst sector was last written to or writing the second data if thesecond sector was last written to.
 21. The computer program product ofclaim 18, wherein the predetermined priority comprises writing the firstdata if the first storage array was last written to or writing thesecond data if the second storage array was last written to.
 22. Thecomputer program product of claim 18, wherein the second executableportion further comprises an executable sub-portion for discarding thefirst data and mirroring the second data if the first storage array waslast written to or discarding the second data and mirroring the firstdata if the second storage array was last written to.
 23. The computerprogram product of claim 18, wherein the predetermined prioritycomprises writing and mirroring the first data if the first data storageblock was last written to or writing and mirroring the second data ifthe second data storage block was last written to.
 24. The computerprogram product of claim 18, wherein the second executable portionfurther comprises discarding the first data and mirroring the seconddata if the first data storage block was last written to or discardingthe second data and mirroring the first data if the second data storageblock was last written to.
 25. The computer program product of claim 18,further comprising: a fourth executable portion for, after the one ofthe first data and the second data is written to both the first datastorage block and the second data storage block, writing the otherrespective one of the first data and the second data to both the firstdata storage block and the second data storage block when the firstwrite command and the second write command are received substantiallysimultaneously